Changing tube on Bravo V1 headphone amp

[cgutz]

Fellow Ak'er andyecon gave me a great deal on a Bravo V1 headphone amp. This is the version with the EH6922 tube. For what it is, it sounds great with my Sennheiser HD424's.

He also sent in the box a 12AU7 tube, which is used in the Bravo V2.

Out of curiosity, I swapped the 12AU7 in. It sounded horrible, but after reading a Rockgrotto thread on the Bravo, I checked bias, it was very low. I adjusted the pots to bring it up to 15v per channel. Then it sounds really great with the 12AU7. It seems less sterile and warmer than the EH6922 through the Sennheisers.

Here is my question, and no amount of "googling" seems to provide an answer - do I risk damaging or harming anything by using the 12AU7 in a Bravo V1 (EH6922 version) with the bias readjusted accordingly? Or would I need to make other mods to continue to use the 12AU7?

FYI the IRF630 Mosfet heatsinks aren't too hot to the touch after using for an hour or more.

 

[elliott66]

The 12au7 is a 12.6 vac filament tube, the 6922 is a 6.3 vac filament tube. I'm not familiar with the Bravo V1 but if you want to continue running the 12au7 you would need to re-wire the filament so that pins 4 and 5 are tied together and connected to one side of the filament supply, and pin 9 connected to the other. This changes the 12au7 to 6.3v filament operation.

And after you've done that who knows if the bias point is in the linear region of the tube? The cathode and plate loads are probably not the same, and if it's a ccs (which I assume since you mentioned changing the bias and a mosfet) the proper current would need to be set.

If the Bravo V2 is built for the 12au7 maybe you could find a schematic and modify the V1 to match?

Otherwise you could try different brands and vintages of the 6922 / 6dj8 / E88CC types. Beware though, tubes of this family are prone to be microphonic.